Fig. 1. Flow chart of feature line detection

Fig. 2. Normal vector according to adjacent points

Fig. 3. Azimuthal angle of normal vectors on projection plane. (a) Point at boundary; (b) point in plane

Fig. 4. Maximum difference of azimuthal angles of adjacent normal vectors. (a) Under different range resolution; (b) under different neighborhood points

Fig. 5. Triangles composed by detection point and nearest neighbor points

Fig. 6. Cluster results of fold point detection. (a) Cluster of point normal vectors in plane; (b) cluster of point normal vectors on fold; (c) cluster of point normal vectors on curve

Fig. 7. Extraction of boundary. (a) Original point cloud; (b) improved RANSAC method; (c) proposed method

Fig. 8. Fold lines extracted by different methods under condition of plane construction. (a) Original point cloud; (b) based on curvature method; (c) based on plane method; (d) based on proposed method

Fig. 9. Fold lines extracted by different methods under condition of non-plane construction. (a) Original point cloud; (b) based on curvature method; (c) based on plane method; (d) based on proposed method

Fig. 10. Point cloud of bridge

Fig. 11. Extracted feature lines of bridge. (a) Feature lines extracted from first phase point cloud; (b) feature lines extracted from second phase point cloud